Insert newline using Unicode scalar to handle non-ASCII newline characters

When inserting a newline character, use the Unicode scalar value instead of a
literal "\n" string. This ensures the correct newline character is inserted
even if the system uses a non-ASCII newline like CRLF.

When generating attributed string for services menu, if an attachment contains
plaintext file contents, replace the attachment with the inlined text content
in the output string. This allows passing the full content, including from
attachments, to services expecting plaintext input.